problem creating rtree index on timestamptz

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Hello,

    I was trying to create a functional index on a timestamptz column and=20
had the following problem. It seems subraction of an interval from a=20
timestamptz is not immutable. Should this be the case?

foo=3D> create table timestamp_tz_test ( start timestamptz primary key );
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index=20
"timestamp_tz_test_pkey" for table "timestamp_tz_test"
CREATE TABLE
=A0
foo=3D> create INDEX idx_timestamptz_rtree on timestamp_tz_test using=20
rtree ((start - '15 minutes'::INTERVAL));
ERROR:  functions in index expression must be marked IMMUTABLE


    I then tried a similar task with just at timestamp column and got this=20
error:

foo=3D> create table timestamp_test ( start timestamp primary key );
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index=20
"timestamp_test_pkey" for table "timestamp_test"
CREATE TABLE
=A0
foo=3D> create INDEX idx_timestamp_rtree on timestamp_test using rtree=20
((start - '15 minutes'::INTERVAL));
ERROR:  data type timestamp without time zone has no default operator=20
class for access method "rtree"
HINT:  You must specify an operator class for the index or define a=20
default operator class for the data type.

Should there be a default for this?

Thanks in advance,

- --hussein
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